Dry kiln and the art of kiln drying



July 26, 1938. .1. F..COBB

DRY KILN AND THE ART 0F' KILN DRYING Filed April 26, 1952 5 Sheets-Sheet 1 r. i :2 H m: x A a a if. a rm L (m m 35H;llitlflimmm--iiNiiiili .33 m: m v ,9 mm mm M2 \& 4 1v H m: 1. th. 1 b A q 8% QS N, mv\k July 26, 1938. J. F. COBB DRY KIL N AND THE ART OF KILN DRYING Filed April 26, 1932 5 Sheets-Sheet 2 I! If I l u n mm mu n same 000 m 2. \H n3 NM |m \/M M. k Juv Q i u w v R m Q 0 Q wig 4 //A/4/A/// '74, w 93 A E. rlL 5 Q Juiy 26, 1938. J. F. CQB'B DRY KILN AND THE ART OF KILN DRYING 5 Sheets-Sheet 3 Filed April 26, 1932 3 N 3 NM at ww 3 mw\ July 26, 1938. J. F. COBB DRY KILN AND 'THE ART OF KILN DRYING Filed A il 26, 1932 5 Sheets-shat 4 .fllllllilfl IIIIIIIII all. I .I!

July 26, 1938. J. F. COBB DRY KILN AND THE ART OF KILN DRYING Filed April 26, 1932 v 5 Sheets-Sheet 5 mzcm \1] INVENTOR. 7 2mm] 4&4

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Patented July 26, 1938 UNITED STATES- 2,124,845 nay AND THE-ART or KILN name James Forrest Cobb, Portland, reg., assignor to Moore DryKiln Company of Oregon, 9. corporation Application April 26, 1932. Serial No. 607,651

6 Claims. (01. 34-46) My invention relates to improvements in dry kilns and in the art of kiln drying.

An object of my invention is to more effectively and efliciently control the drying process.

Another object of my invention is the improvement in the art of drying which .consists of employing a greater rate or volume of kiln atmospheric circulation across the surfaces of the stock being dried at an initial stage of the drying proc- 10 ess and employing lesser rate of circulation at a later stage of the drying process.

Another object of my invention is to accomplish better kiln drying of lumber products and other colloidal substances by .the reduction of the circulation rate across thestock to be dried after the fibre saturation point of the stock has been reached.

Another object of my invention is to increase the efliciency of drying by employing a faster circulation rate of kiln atmosphere across the stock while the drying capacity of the circulating atmosphere is less, and employing a lesser rate 01' circulation across the stock at a later stage of the drying process when the drying capacity of the circulating atmosphere is greater.

Another object of my invention is to employ novel means for effecting a greater rate of circulation at an earlier stage of the drying process, and a lesser rate of circulation at a later stage of the drying process.

Another object of my invention is to efiect a correlative control of temperature, humidity and circulation of the atmosphere within the kiln at the initial and at the final stages of the drying 5 process.

Another object of my invention is to provide a novel variable speed drivemeans for a rotative shaft.

Another object of my invention is to provide a novel means of changing the rate of rotation of a driven member.

Another object of my invention is to provide a novel air circulating system for a dry kiln.

Another object of my invention is to provide a novel heating system and method of its control for a dry kiln.

Another object of my invention is to eflect zone control of heat, humidity and circulation within a kiln.

Another object of my invention is to effect novel zone control or two-stage drying in a progressive dry kiln.

' Another object of my-invention is to control the diflerential of temperature and humidity bes tween a control point or points at one end of the kiln and the temperature and humidity remote from the control point or points at the other end of the kiln.

Another object of my invention is to control the rate of circulation within the kiln according 5 to the deviation of the relative humidity or the wet bulb temperature within the kiln from the set point of the controller.

When lumber dries it shrinks, and if the outside shell of the stock dries too fast it shrinks 10 over a comparatively wet inter or that has not shrunk. As a result, the outsi e surface of the stock develops enough crack or check to make up the diiierence in shrinkage, thus damaging the lumber. v.This is particularly evident at the ends 15 oi the stock, which naturally dry faster thanthe other parts.

Also, the moisture in lumber is classified into two kinds known as "free moisture and "hygroscopic moisture, the flrstbeing contained in'the 20 cell cavities like honey in a comb, and the second being absorbed by the cell walls. The free moisture transfuses freely and may be removed rapidly, and until it is removed from the surface of face, where it may be wiped 03 or absorbed by 30 the surrounding or circulating atmosphere. This dififusion rate is lessthan one half as great as the transfusion rate of the free moisture from the stock.

If the drying rates are made to conform sub- 5 stantialiy to the transfusion and the diffusion rates of the stock respectively, at the earlier and later stages, then the drying progresses evenly, both over and through the pieces of lumber and no casehardening and checking occurs. 40

The drying rate of a kiln atmosphere is a function of its humidity, temperature and circulation. Heretofore, the proper regulative adjustments of humidity and temperature only 7 have been given consideration, and kiln opera- 45 tors have in the past endeavored to solve the 'drying problem involved with oneof the factors missing,namely a properly controlled circulation. K

What I have done is to make use of not only 50 temperature and humidity in the equation, but also of the factor of circulation, rate or volume of air per minute, which gives a useful result that is entirely new.

I have discovered that it is impossible to control the drying rate of stock below the fiber saturation point adequately and economically by varying either the relative humidity or temperature, or both, of the drying atmosphere. Best results can be obtained by a proper control of relative humidity and temperatures and, in addition, a proper control of the rate or volume of circulation. When a proper reduced rate or volume of circulation is used during the latter stage of the drying process, the removal of moisture from the surface of the stock is made to substantially conform to the diffusion rate, and a material improvement in the quality of drying results therefrom. Relative humidities of the circulating atmosphere, lower than in the old practice, may be used during this stage because of the controlled circulation. In other words, when drying out, specifically, the hygroscopic moisture, best results are obtained by maintaining a practicable degree of relativity betweenthe temperature necessary to effect-diffusion, and the humidity of the drying atmosphere, and also a reduced rate of circulation across-the stock. Diflerent woods 'will require variations of all three factors, and" hence it is not possible to give even a general table or rule relating thereto'.- However, a skilled dry kiln operator, being supplied with the underlyingidea, will be able to apply the proper variation in any given case.

What constitutes my invention will be herein after specified in detail and succinctly defined in the appended claims.

In the accompanying drawings wherein my invention is illustrated, I present in preferred form of embodiment,

Figure I is a partial horizontal cross section, of one of my kilns taken ina plane above the floor, with piping connections omitted.

Figure'II is a partial longitudinal vertical section partially diagrammatic, taken along the lines II-II of Figure I.

Figure III is a transverse section taken along the lines III-III of Figures I, II, VIII and IX, illustrating stock loads within the kiln.

Figure IV is a transverse vertical-section taken along the lines IVIV of Figures I and II.

Figure V is a plan view of a motor and drive mechanism which may be used for one of my kilns.

Figure VI is a front elevation of Figure V;

Figure VII is a side elevation of Figure VI.

Figure VIII is a partial horizontal section of an alternate design of kiln to that shown in Figure I illustrating features of my invention, with piping connections omitted.

Figure IX is a partial longitudinal vertical section partially diagrammatic, taken along the lines IX-IX of Figure VIII.

Figure X is a side elevation of an alternate design of heating system for progressive kilns to that illustrated in Figures I and II. I I Figure In is a side elevation of another alternate design of heating system.

Figure XII is a cross sectional. detail of 9. diaphragm valve employedin Figure IX.

Figure XIII is a cross sectional detail of a motor lever employed in Figures VIII and IX.

Figure XIV is a partial detailcross section of the jack screw.

Figure XV is a detail of motor switch for changing the fan R. P. M. of the kiln.

Referring to the numerals on -the drawings, l illustrates the floor of the kiln, 2 and 3 the side walls, 4 the roof and 5 the partition floor. The functions of the partition floor may-be accomsuitable supports such as beams l5, which are 10 preferably supported by the walls 2 and 3, and which may support the partition floori.

The stock loads 6 and I are preferably stacked in layers having stickers l6 between layers ll of culation within the kiln. The stock is preferably stacked edge to edge between stickers on the stock loads, when cross circulation is used, but it is to be understood that the stacking may be of any preferred form and this invention isnot limited go to any particular kind of stacking, or to any particular direction of air circulation within the kiln.

The difference between so-called charge and stock, providing interstices I8 open for air cir- 15 progressive dry kilns is in the manner of their g operation. I preferably provide a kiln which may be operated progressively-that is, a kiln in which green stock is put into the green end at periodic intervals and dry stock is removed from the dry end at periodic intervals. In progressive kilns the go circulation rate and the schedule of dry conditions is varied within the kiln from one end to the other. This principle may also be applied to charge kiln operation in which each kiln is filled with lumber or other material tobe dried, and the u drying conditions are varied from the beginning to the end, during the drying process. My invention relates to themethod of drying stock in both progressive and charge type kilns.

I have illustrated in Figures III and IV single 40 track kilns; however, it is understood that multiple track kilns may be employed. It is also understood that the fans located below the tracks within the kiln, may be a prefrred'form of embodiment.

- F 45 In progressive kilns I preferably provide zone control of temperature, humidity and circulation, considering, for example, that there may be preferably two zones in the kiln-preferably one at each of its ends. In some cases two zones may be employed.

In progressive kilns I preferably provide a greater rate and volume of circulation across the stock at the green or entering end of the kiln more than 50 to evaporate the free water which transfuses freely .to the surface. At this end I also preferably provide a. relatively low temperature and a relatively high humidity In the green end zone when a lower temperature and a higher humidity are employed, the drying .capacity of this at- 60 mosphere is lower and a relatively higher rate ofcirculation is essential. At the dry end of the kiln, which the stock reaches after itis dried to the fibre saturation point, and the stock contains only hygroscopic moisture, I preferably provide a lower rate and volume of atmospheric circulation, also a-higher temperature and a lower relative humidity. The lower rate of circulation in the dry end zone is practicable because the drying capacity of the atmosphere is greater. The reduced rate of circulationat this latter stage also increases the quality of the drying by reducing the tendency to caseharden, reducing the loss of steam. through use of steam humidifying 7 sprays, and reducing the power consumption for unnecessary circulation.

When chargekilns are used I preferably providea faster rate of circulation at the initial stage of the drying process, together with a higher humidity and lower temperature, for the same purpose as described for the green end of progressivekilns, and at a later stage, after the fibre saturation point of the stock has been reached, I reduce the rate. and volume of air circulation across the stock, increase thetemperatg re and reduce the humidity for the same purpose as described for the dry end of progressive kilns.

When reference is made to the green end of a progressive kiln'or the earlier stage of the drying process in charge kilns I refer to the stage of drying in which the stock contains free water as well as hygroscopic moisture. When reference is made to the dryend of a progressive kiln. or to the latter stage of the drying process inacharge kilns, I refer to that stage of the drying process in which there remains only hygroscopic moisture within the stock.

I provide a plurality of heating systems IQ- A,

22B within the kiln supplied with steam through a supply pipe header 23 andhaving drain headers 24 with suitable connections and traps not illustrated for trapping the condensation from the heating pipes.

The headers 23 of the heating systems I9A, 49-3, 2 l--A and 2 I-B at the green end are fed through supply pipes 8--A and 8B respectively from a source of steam supply not illustrated, and the headers '23 of the heating systems 2ll- -A, 20-'B, 22--A and 22B at the dry 'end of the kiln are fed through the supply pipes H-A and I I--B from a source of steam supply not illustrated. Each of the bottom headers 24 is drained and trapped in any usual manner not illustrated.

The A and B units of each heating system are individual units. Either or both of them may be used to provide the amount of heat at each end of thekiln that may be required to maintain the temperature schedule. The "A and B units of each heating system are preferably individually and independently controlled. either by automatic control instruments or manually.

In progressive kilns I preferably employ greater heating capacity at the dry end than at-the green end of the kiln, as illustrated in Figures I and II. In Figure I the heating units |9A, l9-B, 2 l-A and v2ll3 at the green end preferably have less heating capacity than the heating units 20A,

20-3, 22-A and 22-3 at the dry end of the kiln.

In some cases I employ graduated heating systems extending the full length of the kiln with greater heating capacity at the dry end than at the green end, and auxiliary heating units at the green end, at thedry end,-or at both ends for individual control of temperature at the two ends of the kiln. Examples of such heating systems are illustrated in Figures X and XI.

The heating system illustrated in Figures X and XI may be used in progressive kilns when a temperature difference at the two ends of the kiln is desired. ,The top line of the heating system 25 extends preferably the full length of the kiln, and the second, or the second and third lines of the heating system 28 are located at the 'dry'end of the kiln only. Experience has shown that in drying some classes of stock the relationship of not less than twice the heating capacity at the dry end, than at the green end, gives best results in progressive kilns. I preferably provide auxiliary heating units28 at the green end, as illustrated in Figures)! and XI, also auxiliary heatingunits 21 at the dry end of the kiln. The auxiliary units 26 and 21 are preferably separately controlled and may be used toraise the temperature at the green and dry ends respectively of the kiln, or if the heating units are in use, they may be cut on at either end to reduce the temperature.

When only separately controlled heating units of substantially the same capacity are employed at the green and dry ends of the kiln, as illustrated in Figures VIII and IX, I may maintain such temperature difference between the two ends as desired by varying the amount of heating surface in use at the two ends of the kiln, and by utilizing the heating units at the dry end during longer periods of time than at the green end.

When charge kilns are used'I preferably employ a uniform temperature the full length of the kiln to give uniform drying. This. may be accomplished by employing separately controlled heating systems at each of the two ends of the kiln or by employing a single heating system extend ing the full length of the kiln on one side and two independent ventilators at the two'ends respectively of the kiln. The controllers may be set to provide the same drying conditions at each end, or they may be set to maintain slightly different drying conditions if desired to provide a faster drying rate of wetterlstock, for example, that may be located at one end of the kiln, or one of them may be set tomaintain a slow drying I rate for drier stock which may be at one end of a kiln charge.

In kilns in which I employ separate air circulating systems and having two-speed control of the rate of rotation of the fans and having the same size of fans at each end of the kiln, and indi-,

vidual heating systems, ventilating systems, fresh air systems and humidifying systems at each of the two ends respectively of the kiln, I may use the kiln for charge operation or for progressive operation, in either method of operation of which, a faster rate of circulation, a lower temperature,

and a higher humidity, for example, may be maintained at first stage of the drying process, and a lesser rate of circulation, a lower humidity, anda higher temperature, for example, may be maintained at a later stage of the drying process I by separately controlling the systems at the two stages. 1

By having thefacilities above described I am able to maintain the desired conditions for flexible control of temperature, humidity and circulation to give two stage drying for either-charge type or progressive kilns.

In a charge kiln I preferably employ separate Q controls for the heating, humidifying. and air circulating systems at each end of the kiln. For example, a separate temperature and humidity recorder controller is preferably used having thermostatic bulbs at each end of the kiln. and a controller for effecting different rates of rotation of the fans is preferably employed at each end of vthe kiln. These controls are .preferablyset to maintain the same conditions 'of temperature,

humidity and circulation at each end of the kiln. 5 If the temperature, or humidity of the atmosphere of the kiln at the bulbs varies at one end in relation to that at the other end or from a predetermined set point of dry or wet bulb temperature of the controller at that end, caused, for example, by difference in the drying rate of the stock at that end of the kiln in relation to the drying rate at the other end, he controller at that end will automatically ac ate the heating and/or humidifying, and/or air circulation systems at that end. For example, if the dry bulb temperature of the atmosphere is low at one end, caused by faster drying rate of the stock at that end, the controller will open, the diaphragm valves 34, forexample, to supply steam to the pipes 2| until the temperature is raised to the set point. If, for example, the wet bulb temperature of the atmosphere is low at one end of the kiln in relation to that at the other end, the wet bulb controller will actuate the fan speed control to cause the rate of rotation of the fans to be reduced at that end and/or the humidifying sprays to be turned on and/or the ventilators to be closed. When the wet bulb temperature of the circulating atmosphere is raised to the'set point,

the rate of rotation of the fans will automatically be increased to high speed, and/or humidiiying sprays turned off and/or the ventilators opened.

If the moisture content of the stock is greater at one end of the kiln than at the other end, under equal conditions of heat, humidity and circulation, the end having the greater moisture content will require more heat and a greater rateof circulation because there will be a tendency towards a lower temperature and a high humidity at that end. The end having the lower moisture content stock will tend to have a higher .temperature and a lower humidity, which would require less heat and a less rate of circulation if the wet bulb temperature drops below the set point of the controller at that end.

The automatic controllers 30 and II are preferably of any well-known type, each controller having thermostatic dry and wet bulbs within the kiln. For examplejthe controller 30 has a dry bulb I2 and wet bulb 36, while the controller 3| has a dry bulb 33 and a wet bulb 31. The dry bulbs 32 and 33 control, the dry bulb temperature at the two ends of the kilnby actuating the controllers 30 and Il respectively'to'open diaphragm 5 valves 14 and 35 respectively, by air pressure to the controllers through the pipe. I95 from an auxiliary air.supply not illustrated, through the air pipes I00 when the dry bulb temperature within the kiln. at' the two ends drops below the 60 set points.

65 supply notillustrated through the air pipes iiil,

when the'wet bulb temperature within the kiln at the two ends respectively, drops below the set points. The wet bulbs 36 and 31 are preferably I kept wet by wicks 4| an water boxes 40 by a sup- 7 ply of water from the pipes lz from a source not illustrated. 1

'The circulating system preferably comprises rotary driven fans, for example, 45 and ,bperatively mounted on one or more shafts 11, or 41 75 and #8, carried on suitable bearings 48, which are supported by hearing stands 80. The bearings 4! and thrust bearing 60, are provided with lubrication preferably through 011 cups 5|, which may be located within or outside of the kiln. The fans 45 and 46 may be either right hand or left hand, but they are illustrated as left hand fans 45 and right hand fans 46. The fans .45 are illustrated of larger diameter and having larger circulating capacity than the fans 46. Fans 45 are located at the green end of a Progressive kiln and the fans 46 are located at the dry end of a progressive kiln (see Figures I and II for example). If

' charge operation is employed, it is understood that the fans may be of substantially the same circulating capacity, for example, and are designated l6 and II in Figures VIII and IX, but will be rotated at different rates of speed at diflerent ends of a progressive kiln, or rotated at the same R. P. Mrat high speed at the beginning of the drying process in charge kilns, and at the same R. P. M. at low speed at a later stage of the drying process in charge kilns, to'accomplish different rates of circulation across the stock at difl'erent stages of the drying process in both progressive and charge type kilns.

-The air circulating system shown in Figures I and II is that described in Patent #1,680,013, issued August 7, 1928. However, it is understood thatthe principles andfeatures of my present mounted in any other manner so long as they eflect a circulation across stock which may be stacked in any preferable manner within the kiln.

A fan shaft 41, in Figures I and II, is rotated by suitable driving means located preferably outside the kiln, such as, for instance, an electric motor 54 and a belt 55. Means are provided for reversing the circulation at will, such as by any well-known type of reversing switch not illustrated.

I preferably maintain independent drying conditions at the green end and dry end zones of a progressive kiln. This is accomplished by employing independent heating, humidifying, circulating-and ventilating means at the green and dry end zones of the kiln which may be independently and individually controlled.

In charge type kilns I preferably provide a variable speed driving means forkilns, which may be an individual variable speed motor rotating the fans at each of the two ends of the kiln, or constant speed motors 58, or 58 and 59, together with other means comprising stepped pulleys.v The stepped pulley on the motor 58 has two driving faces, one SI of larger diameter, and the other 62 of smaller diameter. The driven pulley comprises a face 64, which is of smaller,

diameter and is aligned to be driven by the driving pulley ii. The other face 65 of the driven pulley is aligned to be driven by the smaller driving face 62 of the'driver pulley.

I preferably provide stepped pulleys which will give, for-e'xai'nple, a reduced fan shaft R. P. M. at the later stage of the drying process, requiring about half as much motor horsepower asthe reater R. P. M. of the fan shaft at the initial stage.

A motor 59 and its variable speed mechanism, similar to and having the same characteristics as the motor 58 and its variable speed mechanism, is preferably provided at the opposite end of the kiln. v

. The motors 587 and 59 are mounted on suitable bases -which are pivoted. on one edge adjaon the pivoted plate toward or away from the hinge pin 14. For example, it may bemoved toward the hinge pm by loosening the bolts 16,

and by loosening the screws 11, and by tightening the screws '18 against the opposite side ,of

the motor.

In addition to the weight of the motor, I preferably provide means such as springs for maintaining belt tension. For this purpose I provide a threaded rod 70, or other means, which is hinged or otherwise attached to a clevis 8| on the rigid base 12, or on its supports 1|. Tension is transmitted to the belt through a bracket member 82, which is attached to the pivoted plate 13, to which the motor is fastened, by means of a thumb jack screw 83, having a shoulder 84,

and a collar 85. A suitable opening 86 in the pivoted plate is provided to'allow free turning of the jack screw 83 in the pivoted plate. The jack screw is threaded into the bracket 82.

The bracket 82 may be of any shape, but is illustrated in the form of a -Z and has means for attachment to .the pivot plate, suchas an opening 88 at one'end and means connecting with a spring 'at the other end, such as an opening .81. The opening 63 at one end is threaded for the jack screw 83, and the opening 81 at the other end is large enough to pass freely over the threaded rod (8. I

The threaded rod Hi-passes through the opening 81 in the bracket 82 and has two springs 19 and 80,. one on each side of the bracket 82 being separated therefrom by-washers 88. Washers 89 separate the springs 19 and 80 from the nuts 90 and 9| respectively, which are threaded on the rod 10. By adjustment of the nuts on the threaded rod lll, change in the tension of the springs I9 and 88 is effected. By adjusting the nuts 90 and 9| upwardly, the drive centers of the driver and driven pulleys are reduced. By adjusting the nuts 99 and 9| downwardly, the

drive centers of the driver and driven pulleys are increased.

The ordinary type of pivoted motor base does not permit a ready reversal of the motor. It works well in .one direction of rotation when the tight side of the belt is adjacent to the pivot point of the pivot plate, but does not work-so well in the reverse direction of rotation when the tight side of the belt is on the side opposite for each'driven pulley speed, to

pin 14 as a pivot. The spring 88 is thus employed to augment the weight of the motor in this direction of rotation in order to maintain a tension on the belt equal to the tension produced by the weight of the motor when the bottom side of the belt is tight and the lever action tending to raise the motor is less.

The means of maintaining uniform belt tension I for either direction of motor rotation overcomes the disadvantage ofthe ordinary type of pivoted 'motor base for reversal of rotation, and I am able to reverse the motors on mykilns without losing the proper belt tension.

The proper adjustment of the springs I9 and accomplish this purpose. The spring 98 is of sufficient capacity to exert a force downward to maintain belt tension when the motor is rotating in a clockwise direction as shown in Figure VI equal to the tension on the belts when the motor is rotating in an anti-clockwise direction-as shown in Figure VI. The springs 19 and 80 are preferably selected to provide sufficient strength for their purpose, there being but little compression on them when properly adjusted. The springs 19 and 80 also permit a relativemovem'ent of the a belt pulleys which aids in maintaining uniform belt tension. The effective weight of the motor for increasing or reducing belt tension may be changed by adjustment of, for example, the nuts and 9|. In fact, this adjustment provides an additional means of regulating belt tension and of maintaining constant belt tension in either direction of rotation of the motor or driven pulley.

I also provide means for facilitating the change of speed of the-driven shaft byshifting the belt 66 from the large drive pulley 6|, and the small driven pulley 84 to the small drive pulley 82, and

the large driven pulley 65, and vice versa. This means may be accomplished by the use, for example, of the thumb jack screw 83, one end of which is attached to the pivot plate 13, and the other end of which is threaded into one end oi the bracket 82. The other end of the bracket 82 is provided .with an opening 81, through which the threaded rod 10 passes. When it is desired to shift the belt. thev jack screw 83 may bescrewed out of or de tached from the bracket 82, and the motor raised. After the shift of the belt ismade, the motor can be lowered by'screwing in'the jackscrew 83 to tighten .up the belt, aided by the force of gravity depending on the weight of, the motor and the strength of the spring 80. Some. adjustment of belt tension may be obtained bythe adjustment of the jack screw.

I preferably provide large and small driving and driven pulleysin such relationship that the'posltion of the motor and the adjustment of the springs willbe relatively the same with either fan speed for any given length of belt. In some cases,

however, it may be necessary -to.use two belts, one accomplish this result. a

It is understoodthat the fans for effecting cirkiln, in which varying rates of circulation may be applied across the stock at different stages of the drying process. This invention is not limited to external fan kilns or internal fan kilns of the designs well-known in the art.

In Figure I illustrating a progressive kiln, I have shown a cross circulation type kiln employing a 70. to'the design of kiln shown, but may be applied zigzag duct system; except that I have disclosed the use of larger fans, or fans having a greater circulating capacity at the green end of the kiln and smaller fans, or fans having a smaller circulating capacity at the dry end of the kiln. When in operation these fans exhaust the atmosphere relative humidity according to a predeterminedamount at different stages of the drying process.

I also provide ventilators I and III, which are separately controlled and mounted in series at the green-and dry ends respectively of the kiln. A series of ventilators I20 and I2I may be subiect to automatic control by the controllers 30 and 3| respectively, being actuated by the wet bulb temperature, for example, or they may be operated by hand to maintain a desired wet bulb temperature at the two ends of the kiln. The ventilators I20 also may be operated in conjunction with the separately controlled fresh air conduits 64 and 66, having fresh air doors 96 and 98, and

the ventilators I2I may be operated in conjunction with the separately controlled fresh air conduits 62 and 93, havingfresh air doors-61 and 99.

The ventilators may be of any well known or preferred type. For example, I illustrate dampers I66 in ventilating stacks I20 and IZI' through the roof 4 of the kiln. The. dampers are mounted on the rods I49 and I60 which may be rotated to ad- Just the ventilator openings at the green and dry ends of the kiln respectively. The ventilating stacks may be of any preferred number and may connect with any portion of the kiln.

Fresh air conduits 64 and 95, having fresh air doors 66 and II respectively, are on two sides of one end of the kiln and fresh ,air conduits 62 and 03, having fresh air doors 01 and 08 respectively,

are on two sides of the other end of the kiln.

I provide suitable ports I" in the fresh air.

conduits, preferably one in each conduit at each fan. These ports may be adjustable to distribute the fresh air intaken into the kiln to the several fans as desired. In cross circulation kilns theiresh air cdnduits on one side only may be used. at both ends of the kiln.

I preferably provide ceiling baiiies I66, also floor tem and the fresh air system within the kiln at diilerent stages of the drying process. These systems may be controlled at the green end of a progressive kiln, or at the initial stage of the drying process to obtain a faster drying rate of the stock at the initial stage of the drying process when the moisture content of the stock is high, and they may be separately controlled at the dry end section of a progressive kiln, or at the later stage of the drying process to maintain a different drying condition for the stock at a later stage in the drying process.

The'controllers 30 and II may be of any well- P rt I00.

known type, preferably 01' the two pen type known as temperature and humidity recorder controllers, such as manufactured by well-known instrument manufacturers, controlling both dry and wet bulb temperatures. One is described in Bulletin #2907 issued by Moore Dry Kiln Company of Oregon in July 1929. The same results can be obtained by using two separate controllers, one i' r the dry bulb temperature and another for the wet bulb temperature. In this case both the dry bulb and wet bulb controllers are'considered as the control instrument 30 or 3i. An example 'of such single pen control instruments is described in the Patent #1,-i05,181 issued to E. H. Bristol, January 31, 1922, and another example is described in the Patent #1,582,868, issued to A. M. Dixon, April 27,

'1926. These instruments preferably employ an auxiliary air supply to actuate diaphragm valves and motor levers to control. the drying condition within the kiln.

The diaphragm valves on both heating pipes and humidifying sprays may be of any wellknown design, such for example, as illustrated in Figure XII, which may be opened with air pressure through the air pipes I00, to supply steam to the heatin'gpipes within the kiln, or the air pipe IOI, to supply steam to the humidifying sprays.

to the valve body I6I by the nut I66. A diaphragm plate I55 isattached to a continuation of the valve stem I66, known asthe valve rod I61, and rests against a rubber diaphragm'lfl, which separates the valve plate I66 from the diaphragm cover I, providing an air tight chamber I Ill between the cover I64 and the diaphragm I66, to

which the air supply pipe I00 is connected. ,A spring III is placed between an adjustable nut I61 on the valve rod I61, and the valve spring sup- The valve spring nut III is adjusted to provide the proper tension so that the valve will close automatically when air is released from the diaphragm chamber III. The valve-may be opened by pressure from an auxiliary air supply through the pipe I00 to'the diaphragm chamber against the compression of the spring Ill.-

The motor levers actuatingthe fresh air conduit doors or the ventilators may be of any wellknown type, for example, suchas shown in Figure XIII. They are shown as closing the fresh air doors and ventilators by air pressure supplied through the pipes I02 and I0; respectively. Figure XIII illustrates the design of motor lever indicated as I06, I01, I00, I00, III, 1,!!! and. "0.

A motor lever, as illustrated in Figure XIII, actuates the fresh air doors, ventilators, and the speed changing mechanism for changing the missus vision for an adjustable weight us on the opposite side of the diaphragm fr from the diaphragm plate so that the diaphragm weight H5 will remove the diaphragm lever I60 when air pressure from the diaphragm chamber I I Iis released. When,air pressure is admitted to the diaphragm chamber through the air pipe I03, the diaphragm lever I60 moves in the opposite direction. The diaphragm lever I60 is connected by a connecting rod I63 to a ventilator arm I90, of a ventilator damper I in a ventilator stack. When air pressure is admitted to the diaphragm chamber III, the diaphra weight lever I6I is lifted and the lever I63 mov to close the ventilators by means of movement of the diaphragm lever I60, the connecting rod I63, and the ventilator arm I94. When the air pressure is released from the diaphragm chamber, the ventilator is automatically opened by means of the I pull of gravity on the weight H5 onthe weight lever I6l.

When the air supply is cut oil to any of the diaphragm valves or motor levers described above, the air pressure in the air supply pipes, also in the diaphragm chambers H and III of the diaphragm valves or motor levers respectivelyis allowed to escape through an air leak in the instrument which is normally provided for this purpose, allowing the springs III or the counterweights II on the diaphragm valves and motor levers respectively to close the diaphragm valves and 'open the fresh air doors and/or the ventilators of the kiln.

For example, when the temperature within thekiln at the position of the thermostatic bulb 32 drops below the set point on the control instrument, air from an auxiliary source is automatically supplied through the air pipe I00 to the diaphragm valve 3| to open the valve and supply steam, for example, to the heating pipes I9--A, ii-B, 2I-A and 2I-B, which heat the kiln at the green end, for example. ,As soon asthe temperature within the kiln at the position of the control bulb 32 reaches the set point on the instrument, the auxiliary air supply is cut off from the air pipe I00 and the air pressure in the pipe I 00 and in the diaphragm chamber I I0 is allowed to leak out through an air leak in the controller 30 normally provided for this purpose.

When the wet bulb temperature within the kiln, for example, at the green end, drops below the set point of the thermostatic bulb 36, the control instrument automatically supplies air from an outside source through the pipe IM to the humidifying spray diaphragm valve 6 to open it and supply steam, for example, to the humidifying sprays I22 at the green end of the kiln, through th pipe I02, to the motor levers I06 or II6 to close the fresh air doors at the green end of the kiln, and through the pipes I03 to the motor levers I06, to close the ventilators at the green end of the kiln.

As soon as the wet bulb temperature within the kiln at the position of the wet bulb 36 at thegreen end rises to the set point, the instrument automatically cuts off the supply of airin the pipe IM and allows the air pressure in :the pipe IOI and in the diaphragm chamber IIO, also the air below the set point.

fresh air door 06 or 30, and the weights III also opennthe ventilators I20 respectively at the'green' end of thekiln.

The above description of the operation of diaphragm valves and motor levers at the green end of the kiln by the controller 30, applies also to the operation of the diaphragm valves 35 and 68 and the motor levers I01, I03 and II! at the dry=end of the kiln by means of the control instrument 3 I. It is understood that the drying conditions at the two ends of the kiln are controlled separately in progressive kilns to maintain a faster rate of circulation and preferably a lower temperature and a higher humidity at the green than at the dry end of the kiln. If an automatic temperature and humidity control instrument is not provided for this purpose, then the control may be accomplished manually.

It is understood that with either direction of air circulation within the kiln only one of the fresh air conduits and its corresponding fresh air door at either end of thekiln may be used in the cross circulation type of kiln, such as illustrated in Figures III, VIII and IX. The same applies to the operation of the ventilators. It is understood that the ventilators on one or both sides of the kiln at either end may be operated at any given time.

In drying refractory woods requiring a very particular drying schedule, I preferably control employ the high rate of circulation. When the relative humidity is lower than the set point there is danger of casehardening and damage to the material being dried. It is easier to maintain the wet bulb temperature setting and the humidity at the surface of the stock if therate of circulation across the stock is reduced. I thus preferablyreduce the'rate of circulation when the wet bulb temperature within the kiln drops also provides anadditional safety factor in drying because in case the steam supply to the humidifying sprays is deficient, the fan speed will be automaticallymaintained at a reduced rate until the relative humidity is again raised to the set point. This manner of operation may be employed at the green or the dry ends of the kiln, or both ends, for example, in charge kiln operation, or I may employ this system of reducing the rate of circulation within the kiln at both ends, for example, in a progressiv kiln when the relative humidity or wet bulb emperature at either end from any'cause falls below the set point In order to accomplish this I preferably employ air motors I26 and I26 for actuating control means for varying the rate of circulation,

This manner of operation according to the fluctuation of the wet bulb temperature from the set point of the controllers 30 or 3I respectively. When the wet bulb temperature within the kiln falls below the set point of controller 30, for example, air is supplied through the air pipe I30 to the air motor I25'to actuate the speed changing mechanism, reducing the motor R. P. M. of the motor58 to low speed. Simult aneously with this operation, the humiditying sprays I22 are turned on and the fresh air doors 36 or 98, and the ventilators I20 are-closed.

When the wet bulb temperature within the kiln is raised to the set point, the air supply to the air motor I25 is cut off, allowing the air pressure in the air pipe I80 and in the diaphragm chamber III of the air motor to escape through an opening provided in the controller 80 for this purpose. The counterweight II5 on the air motor causes the operation of the speed changing mechanism to increase the R. P. M. of the motor to high speed. Simultaneously with this operation, the humidifying sprays I22 are turned off control for the motor through use of a polechanging magnetic motor switch I16, having a contactor I12 for high speed of motor, and contactor I13 for low speed of motor.

Relays I14 and I15 normally closed are employed to insure proper operation 01' the pole changing magnetic switch I18, operating automatically through the electrical switch I28, operated by the air motor lever I25.

I preferably start the motorat full speed at the beginning of the operation by manual starting through the push button station I10,with the motor switch "I set for hand control. The current starts at L2 running through the high speed contactor holding-coil I11, through the push button station I10, through to L--5, thus completing the circuit. This closes the high speed contactor and its auxiliary switches IN and I82, which complete the circuit through the contactor holding-coils, preventing the high speed contactors from dropping out when the push button is released. The auxiliary switches I8I and I82 are mechanically interlocked with the high speed contactor I12, so that the auxiliary switches open when the contactor opens, and close whenthe contactor closes. I

For connecting the'motor for automatic operation from high to low speed, or from low to high speed, I throw the automatic control switch I 1| to the automatic position. When the wet bulb temperature within the kiln drops below the set point, on the controller 30, for example, the air operated switch I28 goes to the right, to the. position for low speed, and it closes the circuit through the contacts of relay I15, allowing the current to flow through the contactor holding coil I80 tor the low speed contact. This current-also goes through the relay I14, which disconnects the high speed contactor Just previous to the closing of the low speed contactor.

when the relative humidity or the wet bulb temperature within the kilnat either or both' pipe I 30, and from the diaphragm "chamber I II is released through the opening provided in the controller 30. The circuit is closed through relay I14, allowing the current to flow through the contactor holding-coil I for the high speed contact. This current, which comes from L2, also goes through the relay I15, which opens the low speed contactor I13 Just previous to the closing of the high speed contactor I12. The current goes through the holding coil I11 for the high speed contact, then back to L3, which completes the circuit.

At any time, whether on hand or automatic starting, or whether the motor is running at low or high speed, the motor may be stopped by pushing the stop button of the push" button station I10.

The air motor I 25 may operate either slowly or quickly in closing switch contacts I28 as the current is carried by auxiliary switch contacts I83 or I8I immediately upon operation of either low speed or high speed contactor.

At the start of the kiln operation, I preferably start the motor manually by the push button station I10, and thereafter the air motor and the automatic circulation changing mechanism. will automatically control the-speed of the motor and of the fans within the kiln according to the fluctuation of the wet bulb temperature within the kiln from the set point of the controller 30 or ii. Provision is also made to disconnect the automatic operation of the air motor and the circula-' tion changing mechanism so that the motor speed and the fan speed may be maintained at high speed at the beginning of the drying process if desired, and or, the speed of the motor and oi the fans may be maintained at low speed at a later stage of the dryingprocess. In other words, I preferably provide means for either automatic or manual variation in motor speed and fan speed within the kiln.

For example, in Figures VIII and IX, I illustrate a fan system, a heating system, a humidifying system, a ventilating system, and a motor speed controller for controlling the speed of the fan system, at each end of the kiln. As the wet bulb temperature in the kiln varies from the de- I20 01' the kiln by admitting-air pressure to the motor levers I08, H8, and I08 controlling the fresh air doors and ventilators respectively. Similarly when the wet bulb temperature has reached or rises above the set point, the humiditying spray I22 is automatically cut oil and the fresh air doors 88 or 88 and ventilators I20 may.be opened, and the motor and fan speed increased to the original high speed by reverse action of the switch.

This method 01 operation reduces the hazard of damaging the stock in kiln drying, in that it more promptly and-eirectively brings the relative humidity to the desired point, because slowing up rate of circulation across the-stock causes an increase of humidity at the surface 01 the stock being dried by removing the moisture evaporated less rapidly thus-increasing the diflerential between the temperature and the humidity at the position of the thermostatic control bulbs within the kiln, and the exit air from the stock load or the temperature and humidity at the surface of the stock remote from the control bulbs.

If desired, thesteam spray may be dispensed ducing the circulation rate to slow speed, while the ventilators and fresh air doors are closed,

and until the wet bulb temperature is raised to the set point on the-controller, when high speed circulation is resumed.

At the beginning of the drying process when the stock contains free water, a considerably faster rate of circulation than it is economical to continue through the drying period, is advantageous for conveying heat to and maintaining proper humidity at the surface of the stock.

.After the fibre saturation point is reached, a lower rate of circulation can be used, giving maximum drying efficiency without penalizing theoperation with an excessive power consumption.

The quality {of drying is improved after the fibre saturation point of the stock is reached, by reducing the rate of circulation to a minimum .that will still give a uniform flow of air across all the stock. During the latter stage of the drying process, the transfusion rate is slow and less circulation is required to transmit heat to the surface of the stock for purposes of evaporation. The reducedcirculation rate thus prevents the formation of a steep moisture gradient within the stock, due to too fast surface drying during the latter stage of the drying process, whichwo'uld be accomplished by supplying too much heat to the surface of thestock, and also the necessity of furnishing more humidifying spray than would be required if the rate of circulation were .not

reduced. s v In kilns there is a difference in the temperature and humidity within the kiln between the point,

or points of control namely, at the controller bulbs, and at the surface of the stock most remote from the control bulbs in the cycle of either transverse or longitudinal circulation, The amount of this difference necessarily depends upon the rate of circulation within the kiln. It

is essential to maintain this differential at a practicable minimum at the different stages of the drying process. At the beginning of the drying process a considerable volume of aircirculation within the kiln is necessary ton'educe this difl'erential to this practicable amount. At a later stage of the drying process when the'evaporation rate from the stock is low, considerably less circulation may be employed and yet maintain this differential within this practicable amount. In

- other words, variable speed circulation is employed during the drying process to prevent. large changes in this differential at the different stages, and also to give the desired accuracy of control in the drying of the stock at the different stages. M

An economicaladvantage in the operation of my kilns may be gained by the employment of means for varying the speed of the fans at dif-' ferent stages of the drying operations. In that regard, I have discovered that more driving force is necessary to effect increased rate of circulationpf the drying atmosphere of the kiln when the stock loads are first introduced into the kiln in a wet orlgreen state, that when afterwards they have given oil 'a portion of. their moisture content. The reason appearsto be that the gaseous medium of circulation, in passing through the interstices of the stock loads, takes i up so much'of the moisture therefrom in passing increase of the moisture content'thereof.

Dec. 13, ,1932.

other words, the drying atmosphere of the kiln functionates to better advantage in passing it does in'passing through the'remainder of the distance. The remedy for that condition is to increase the rate or volume of circulation during such periods as the stock is wet and the condition above indicated continues. This requires increase of power for such'period. After the stock becomes 'dr'ir', the reduction of its moisture content diminishes" gradually the necessity for the higher rate of circulation, and permits reduction of the rate of speed of circulation and of the power necessary to effect it. In other words, the rate of speed of circulation is by my invention made proportionate to the lag in the capacity of the circulating medium to absorb mois,

ture in passing across the surfaces of the drying stock loads. The term lag is, in this connection, defined to be loss of the functionatingcapacity of the drying atmosphere, taking into consideration that such capacity is diminished by loss of heat from the drying atmosphere, as well as by The rate of circulation desired is that which will render the drying operation on the stock loads as uniform as possible, throughout its diiferentstages. 1

Hitherto, a substantially constant rate of speed has been imparted-to the fans throughout the whole period ofthe drying operation of the kiln. My invention contemplates. economy in .the saving of power, for instance, through the employ-- ment of means for-driving the fans at diiferent speeds or of interrupting the operation of the fans altogetherzand relying in such case upon natural .draft for effecting circulation in differe'nt stages of the drying operation, proportionate to the condition, at any stage, of the drying. atmosphere, as affected, for example, by the stock to be dried. My invention in this regard, consists inusing fans of variablespeed and in setting their speed proportionately to the amount of work demanded of them at different stages of the drying operation.

It is deemed unnecessary to give a further description of the operation of my invention. However,'it may be said that by employing greater volume of air circulation at the beginning of the drying process, and a lesser volume of air circulation at a later stage after the fibre saturation point of the stockhas been, reached, I-am able to effect a corresponding change in the effectiveness of the heatingsystem, humidifying system and ventilating system, which latter includes both fresh air intake and air outlet from the' kiln. The greater volume of air circulation during one stage produces a greater heating effect from the heating system, produces a greater huthe stock and produces a greater amount of ventilation by accelerating the action of the fresh air portsand ventilators. Correspondingly, a reduction in the rate of circulation at a later stage reduces the effectiveness of each of the above systems.

It is intended that this application be a continuation in part of application Serial #204,022

filed July '7, 1927., now Patent #1,891,060 issued It is understood that thisinvention is not limited to the exact design and form illustrated and described.- Modifications of the principles herein described and; mechanical equivalents are selfevident to those skilled in the art; therefore, it is to be understood that the invention includes within its scope whatever changes fairly come within either the terms or the spirit of the appended claims. v Having described my invention, what I claim i 1. In a dry kiln having enclosure defining walls, meansior the accommodation of stock loads stacked therein, means for circulating kiln atmosphere therein, said means comprising fans operatively mounted therein and distributed throughout the length of the kiln, the fans at one end of said kiln being of less air circulating capacity thanthe fans at the opposite end of said kiln, heating means within said kiln, said'heating means being of greater heating capacity'at the end oi. the kiln having fans of less air circulating' capacity, and being of lesser heating capacity at the'end of the kiln having fans oi greater air circulating capacity.

2. In a' dry kiln, a series of fans .operatively mounted on a longitudinal shaft and distributed thereon substantially the full length of the kiln,

the fans at oneend being of larger diameter than the fans at theother end of the kiln, a

, heating system extending in part the full length of the kiln, said heating system having greater heating capacity at the end havingsmaller fans, and less heating capacity at the end having larger fans.

3. In a progressive kiln having green and dry ends, and stock supporting tracks therein suitable for accommodating material to be dried, a return bend heating system in part extending the full length of the kiln, individual heating means, ventilating means and humidifyi fl means disposed at one end oi the kiln, air circulating means comprisingfans operatively distributed substantially the full length of the kiln, said fans having greater air circulating capacity at the green end, and lesser air circulating capacity at the dry end thereof, said return bend heating system having more heating surface at the dryendofthekiln.

4. In a dry kiln, a'separate air circulating system comprising fans at each end, a separate heating system at each end, a separate humiditying systemat each end, means for changing the speed of rotation of the ians of said air circulating systems, controllers for said heating and humidifying systems, said controllers comprising means for actuating said speed changing means, whereby the speed of rotation of the fans is increased when the humidity adjacent thereto is equal to or higher than a predetermined amount and the speed of rotation of the fans is reduced when said humidity is lower than a predetermined amount. v

5. In a dry kiln, an air circulating system comprising fans therein distributed substantially itsi'ull length, a separate heating and humidiiying system at each end of the kiln, means for changing the speed of rotation of the fans of said air circulating system, a controller for said heating and humidiiying systems formaintaining a predetermined humidity, said controller comprising means for reducing the speed of said fans, when the humidity is lower than the pre-- determined humidity and increasing the speed or the fans when the humidity is equal to or higher than the predetermined amount.

6. In a dry kiln, a separate air circulating system 'at each end, a separate humidifying spray at each end, a controller for the wet bulb temperature at each end, said controller comprising means for reducing therate of air circulation at each end and adding humidity by opening said 'humiditying spray when the wet bulb temperature is lower than a predetermined amount and for increasing the rate of circulation oi said air circulating system and closing said humidifying 4 spray, when the wet bulb temperature is at or 40 

